Refrigeration apparatus

ABSTRACT

An extra low temperature refrigeration system consisting of a two-stage compressor that has an electronic controller. The electronic controller controls an electric expansion valve in the evaporator, which modulates the true superheat. There is also an electric pressure control valve in the condenser, which modulates the head pressure in cool mode, and the suction pressure in defrost mode. The defrost cycle uses a reversing valve. The liquid refrigerant for the inter-stage sub-cooling circuit is constantly provided by a sub-condenser separated from a conventional one condenser system. Therefore, the system provides constant cooling for the compressor motor windings as the compressor runs. There is a one-way pathway from the main condenser circuit to the sub-condenser circuit. This pathway will call for more refrigerant from the main circuit to the sub-cooling circuit as demanded. The inter-stage sub-cooling circuit uses an electric expansion valve true superheat control for its sub-cooler.

This application claims benefit under Title 35 U.S.C 119(e) of U.S. Provisional Application Ser. No. 61/402,657 filed Sep. 1, 2010.

FIELD OF THE INVENTION

This invention relates generally to space cooling systems, in particular to a low temperature refrigeration apparatus for providing space cooling with significant energy and cost savings.

BACKGROUND OF THE INVENTION

A typical space cooling system includes at least one evaporator system contained within the space that is to be cooled, a condenser system that is located outside of the cooled space, and a compressor positioned between the condenser system outlet and the evaporator system inlet and, finally, an expansion valve which completes the loop joining together the condenser system outlet and the evaporator system inlet. A refrigerant is circulated within the loop, which cools the space as follows. The refrigerant is compressed by the compressor, which raises the temperature and pressure of the refrigerant. The hot pressurized refrigerant gas then flows through the condenser system, which serves as heat exchanger to allow the refrigerant to dissipate the heat of pressurization. The refrigerant condenses into a liquid and then flows through the expansion valve, where the liquid refrigerant moves from a high-pressure zone into a low-pressure zone, thus expanding and evaporating. In evaporating, the refrigerant becomes cold where it then passes into coils of the evaporator, thus absorbing heat from inside the space that is to be cooled and the cycle then repeats until the space reaches the desired temperature.

In addition to these major components, additional components are also included. A fan assists the heat transfer from the cooled space to the coils of the evaporator system and another fan is used to assist the heat transfer from the coils of the condenser to the outside environment. A negative pressure differential is present on the evaporator outlet when the device is operating in a refrigeration mode thereby suctioning the gas refrigerant to the compressor. Further, thermistor sensors are placed at the inlet and outlet of the evaporator system for measuring the level of superheat across the evaporator. A sensor located on the outlet side of the compressor measures the discharge temperature of compressor. The ambient temperature of the space to be cooled is measured by still another sensor. Finally, the need to defrost the evaporator from any ice build-up due to the cooling process is determined by another sensor that is associated with the evaporator so that defrosting procedures can be monitored.

However, there is not found in the prior art an extra low temperature refrigeration system that combines the features of dual valve control, dual circuit condenser design, superheat control on the sub-cooler circuit, a liquid receiver to store extra refrigerant charge, an oil-pressure initiated defrost, parallel hot gas drain pan loop design on the evaporator to defrost the drain pan, and a compressor superheat control that will provide performance and protection features and lower head pressure to achieve high reliability and substantial operating cost savings.

SUMMARY OF THE INVENTION

It is an aspect of the invention to provide a refrigeration apparatus that has a dual valve control.

Another aspect of the invention is to provide a refrigeration apparatus that has a two-stage compressor.

It is still another aspect of the invention to provide a refrigeration apparatus that uses a dual circuit condenser design.

Another aspect of the invention is to provide a refrigeration apparatus that uses an electric expansion valve for true superheat control for the sub-cooler.

It is an aspect of the invention to provide a refrigeration apparatus that features a bi-flow liquid receiver and surge protector.

Another aspect of the invention is to provide a refrigeration apparatus that has an oil pressure initiated defrost.

Another aspect of the invention is to provide a refrigeration apparatus that has compressor superheat control.

Finally, it is an aspect of the invention to provide a refrigeration apparatus that utilizes parallel hot gas drain pan loop design on the evaporator.

The invention is an extra low temperature refrigeration system consisting of a two-stage compressor that has an electronic controller. The electronic controller controls an electric expansion valve in the evaporator, which modulates the true superheat. There is also an electric pressure control valve in the condenser, which modulates the head pressure in cool mode, and the suction pressure in defrost mode. The defrost cycle uses a reversing valve. The liquid refrigerant for the inter-stage sub-cooling circuit is constantly provided by a sub-condenser separated from the conventional one condenser system. Therefore, the system provides constant cooling for the compressor motor windings as the compressor runs. There is a one-way pathway from the main condenser circuit to the sub-condenser circuit. This pathway will call for more refrigerant from the main circuit to the sub-cooling circuit as demanded. The inter-stage sub-cooling circuit uses an electric expansion valve true superheat control for its sub-cooler. This accurate cooling of superheat improves the life span the compressor and maximizes the main liquid sub-cooling. The reverse cycle defrost method, which is disclosed in U.S. Pat. No. 7,073,344, permits the defrosting the evaporator system in less time required for conventional defrost methods. The reverse flow during defrost also brings the lubrication oil back to the compressor. This cooling system also eliminates the need for a mechanical head pressure control valve and many check valves typically seen in conventional systems. The lower head pressure operation allows energy savings during low ambient conditions. Also, the system uses a liquid reservoir to store extra refrigerant charge. The liquid reservoir acts as a liquid receiver for cooling mode and a refrigerant surge protection tank for defrosting mode. The system uses an oil differential pressure switch as a digital input to the main controller to initiate defrost for proper oil return when the oil pressure falls below a permitted pressure level. Further, the system uses a temperature-sensing probe mounted at the compressor suction copper tubing to measure the compressor true superheat. The compressor true superheat is controlled by varying the evaporator electric expansion valve opening in order to protect the compressor from overloading and liquid refrigerant flood-back. The evaporator uses compressor-discharged vapor to defrost the drain pan. The drain pan piping is paralleled to the refrigerating circuit piping which is not seen in previous reverse cycle defrost systems. This reduces pressure drop of reverse flow and offers a fast and clean defrost throughout the evaporator coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the most basic embodiment of the invention operating during a refrigeration cycle.

FIG. 2 is a schematic of the embodiment shown in FIG. 1 operating during a defrost cycle.

DETAILED DESCRIPTION OF THE INVENTION Refrigeration Mode:

As shown in FIG. 1, invention utilizes two-stage reciprocating compressor 10. Refrigerant (not shown) is compressed by compressor 10. The temperature and pressure of the refrigerant is raised. The hot pressurized refrigerant gas then flows through the oil separator 42. The refrigerant oil is then separated by 42 and discharged back to the compressor 10 crankcase. Oil return solenoid valve 44 is powered with the synchronization of the compressor 10. When compressor 10 is in operation, solenoid valve 44 is open allowing oil to flow through.

The refrigerant after oil separator 42 is diverted into two directions. The first direction goes to the main circuit condenser 38 through the four way reversing valve 40. The second direction is to the sub-circuit condenser 36.

The majority of the high-pressure refrigerant goes to main condenser 38 and is condensed into a liquid state. Electric pressure control valve 32 is used to modulate the pressure of the refrigerant inside the main condenser 38. The liquid refrigerant then enters liquid receiver/surge protector 28. Sub-cooling heat exchanger 24 is used to cool the refrigerant from upstream while the pressure maintains constant. The liquid refrigerant of cold and high-pressure state flows through bi-flow filter/drier 22 and is metered into an evaporator 14 inside freezer box by electric expansion valve 20. The liquid refrigerant expands and now is in low-pressure state in evaporator 14. The refrigerant absorbs heat from ambient around the evaporator and changes to vapor state. The amount of the refrigerant is modulated by electric expansion valve 20 such that when it leaves the evaporator all refrigerant becomes in the vapor state. The vapor refrigerant goes though four way reversing valve 40 and suction accumulator 12 and then returns to compressor 10 via suction port S.

The amount of refrigerant required for the sub-cooling circuit is determined by the inter-stage sub-cooling capacity of the two-stage compressor 10 and the required cooling temperature. This refrigerant enters sub-condenser 36 and is condensed into a liquid state. The liquid refrigerant flows through filter/drier 30 and is metered into an evaporator of sub-cooling heat exchanger 24 by electric expansion valve 26. After evaporating into a vapor state, the refrigerant returns to the inter-stage of compressor 10. The mixture of the refrigerant from the low stage discharge and the sub-cooling is also used to cool the compressor motor windings. Check valve 34 is used to allow some refrigerant to flow from the main circuit to the sub-cooling circuit when more refrigerant is needed for sub-cooling.

Defrosting Mode:

As shown in FIG. 2, during defrosting mode, four way reversing valve 40 is switched to the defrosting position as illustrated. The high temperature and pressure refrigerant in the vapor state discharged from compressor 10 enters evaporator 14 on the opposite side of the refrigerating mode after it flows through four-way reversing valve 40. A portion of this refrigerant enters drain pan heater loop 16 and melts the ice or frost residing on the drain pan. The major portion of the refrigerant enters the evaporator coil circuit and melts the ice or frost residing within the evaporator coil. The refrigerant in drain pan heater loop 16 flows through check valve 18 and meets the refrigerant from the evaporator 14. Electric expansion valve 20 is completely open allowing refrigerant to free flow back to the compressor. After defrosting, the refrigerant flows through bi-flow filter/drier 22 and sub-cooling heat exchanger 24 and back to liquid receiver/surge protector 28. Electric pressure control valve 32 then meters the amount of refrigerant going through main condenser 38. Main condenser in the defrosting mode is then an evaporator absorbing heat from the outside environment.

The refrigerant in the sub-condenser circuit provides constant cooling to the compressor motor windings as it does during refrigerating mode.

Although the present invention has been described with reference to certain preferred embodiments thereof, other versions are readily apparent to those of ordinary skill in the preferred embodiments contained herein. 

What is claimed is:
 1. A refrigeration system for cooling a space, said system having a refrigeration mode and having a defrost mode, said system comprising: a two-stage compressor to compress a refrigerant; a two-circuit condenser connected to said two-stage compressor, said condenser comprising a sub-condenser circuit and a main condenser circuit; an evaporator connected to said main condenser circuit of said two-circuit condenser, said evaporator absorbs heat from the space that is to be cooled during refrigeration mode; a sub-cooling heat exchanger having a first connection between said sub-condenser circuit of said two-circuit condenser and said two-stage compressor and a second connection that is between said main condenser and said evaporator; a first electric expansion valve connected between said sub-condenser circuit of said two-circuit condenser and said sub-cooling heat exchanger; a second electric expansion valve connected between said sub-cooling heat exchanger and said evaporator; a third electric expansion valve connected between main condenser circuit of said two-circuit condenser; said third electric expansion valve also controls head pressure of said main condenser circuit of said two-circuit condenser during refrigeration mode; means for electronic controlling said system wherein said controller means controls said first electric expansion valve to modulate the true superheat of said sub-cooling heat exchanger and wherein said controller means controls said second electric expansion valve to modulate the true superheat of said evaporator when said system is in the refrigeration mode and wherein said controller means controls said third electric expansion valve to modulate the suction pressure of said main condenser circuit of said two-circuit condenser when said system is in the defrost mode and modulates the head pressure when said system is in the refrigeration mode.
 2. The system of claim 1 further comprising: a differential oil pressure switch associated with said controller means such that said defrost mode is initiated when oil level is low.
 3. The system of claim 1 further comprising: a drain pan beneath said evaporator; a drain pan loop associated with said evaporator, wherein a portion of said refrigerant enters said loop to melt the ice or frost and the major portion of said refrigerant enters said evaporator to melt the ice or frost when said system is in the defrost mode.
 4. The system of claim 1 further comprising a refrigerant reservoir interposed between said sub-cooling heat exchanger and said main condenser in the connection connecting said sub-cooling heat exchanger and said main condenser.
 5. The system of claim 1 further comprising an oil separator connected between said two-stage compressor and said two-circuit condenser such that refrigerant oil is separated by said oil separator and discharged back to a crankcase in said two-stage compressor. 